1. Technical Field
The present invention relates to a DA converter and a DA converting method for converting a digital signal into an analog signal. In particular, the present invention relates to a DA converter and a DA converting method that use delta-sigma modulation.
2. Related Art
A circuit using delta-sigma modulation is known as a DA converter for converting a digital signal into an analog signal. Delta-sigma modulation involves converting a digital signal into an analog signal by performing a feedback process to subtract a prescribed reference value from an added value when the value sequentially added to the level of the digital signal becomes greater than the reference value, as shown in, for example, Japanese Patent Application Publication No. 2001-24512.
FIG. 7 shows a conventional DA converter 200. The DA converter 200 is provided with an integrator 210, a level comparing section 220, an interpolator 230, a digital delayer 260, a level subtracting section 270, and a signal processing section 280.
The interpolator 230 interpolates values between pairs of data points in the input digital signal. The integrator 210 performs integration by sequentially adding together the value of the digital signal. The level comparing section 220 outputs a signal indicating whether the output of the integrator 210 is greater than the reference value. The digital delayer 260 delays the signal output by the level comparing section 220 by one cycle. Here, “one cycle” refers to one cycle of a sampling clock supplied to the level comparing section 220. The level subtracting section 270 subtracts the value output by the digital delayer 260 from the level of the digital signal input thereto, and supplies the result to the integrator 210.
The signal processing section 280 generates an analog signal based on the signal output by the level comparing section 220. For example, the signal processing section 280 may generate the analog signal according to the distribution of the timings at which the level comparing section 220 outputs a logic value of 1.
As described above, the DA converter 200 generates the analog signal based on the timing at which the level comparing section 220 outputs a logic value of 1. Therefore, the DA converter 200 can generate the value of the analog signal with a higher resolution when the level comparing section 220 has a higher sampling frequency and a higher resolution with respect to time.
However, there is a limit to how much the sampling frequency of the level comparing section 220 can be enhanced simply by increasing the frequency of the sampling clock. Furthermore, it is difficult for the comparison result by the level comparing section 220 to be fed back to the level subtracting section 270 within a single cycle of a high-speed sampling clock.